WDPN32 PGTW 221500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 21W (TWENTYONE) 
WARNING NR 001//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 12.8N 113.3E
   INITIAL INTENSITY: 25 KTS
   GEOGRAPHIC REFERENCE: 348 NM EAST-SOUTHEAST OF DA NANG, VIETNAM
   MOVEMENT PAST 6 HOURS: WEST-NORTHWESTWARD AT 11 KTS
   SIGNIFICANT WAVE HEIGHT: 10 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS A
RAPIDLY CONSOLIDATING SYSTEM WITH FLARING DEEP CONVECTIVE BLOOMING
OVER THE ASSESSED LOW LEVEL CIRCULATION CENTER (LLCC) OVER THE PAST
SIX HOURS. A 221121Z SSMIS 89GHZ MICROWAVE IMAGE SHOWED INTENSE
CONVECTIVE BANDS WRAPPING ALONG THE WESTERN PERIPHERY OF THE
DEFINED LLCC. THE INITIAL POSITION IS ASSESSED WITH HIGH CONFIDENCE
BASED ON THE AFOREMENTIONED MICROWAVE IMAGE AND A 221248Z ASCAT-B
SCATTEROMETER BULLSEYE. THE INITIAL INTENSITY IS ASSESSED WITH LOW
CONFIDENCE BASED ON THE T1.5 PGTW CURRENT INTENSITY ESTIMATE AND
THE RAPIDLY IMPROVED STRUCTURE. WHILE THE AFOREMENTIONED ASCAT-B
PASS ONLY SHOWED 20 KNOT WIND BARBS MOSTLY FAR TO THE NORTHEAST, AN
EARLIER SCATTEROMETER PASS INDICATED 25-30 KNOT WINDS SURROUNDING
THE LLCC. ADDITIONALLY, A SHIP OBSERVATION FROM 0800Z WITHIN 40 NM
OF THE ASSESSED CENTER AT THAT TIME INDICATED A PRESSURE OF 1004 MB
EQUATING TO 24-25 KNOTS USING THE WIND PRESSURE RELATIONSHIP.
SUBSEQUENT IMAGERY FROM THE 1400Z HOUR SHOWS WHAT COULD POTENTIALLY
BE THE INITIAL INDICATIONS OF AN EYE IN THE EIR, THOUGH WITHOUT
MICROWAVE IMAGERY IT IS HARD TO CONFIRM THIS AS A NASCENT EYE
FEATURE OR JUST AN AREA OF WARMER CLOUD TOPS. THE SYSTEM LIES IN A
FAVORABLE ENVIRONMENT CHARACTERIZED BY LOW VWS, WARM SSTS AND
MODERATE EQUATORWARD AND WEAK POLEWARD OUTFLOW.

INITIAL WIND RADII BASIS: NOT APPLICABLE (THERE ARE NO INITIAL WIND
RADII).

CURRENT STEERING MECHANISM: SOUTHERN PERIPHERY OF THE LOW- TO
MID-LEVEL SUBTROPICAL RIDGE (STR) TO THE NORTH. 

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T1.5 - 25 KTS

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE
   VWS: 10-15 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: MODERATE EQUATORWARD
   OTHER FACTORS: LIMITED WEAK POLEWARD OUTFLOW. 

ANALYSIS CONFIDENCE:
   INITIAL POSITION: HIGH
   INITIAL INTENSITY: LOW
   INITIAL WIND RADII: NOT APPLICABLE

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THIS INITIAL PROGNOSTIC REASONING
MESSAGE ESTABLISHES THE FORECAST PHILOSOPHY.

FORECAST DISCUSSION: TD 21W IS FORECASTED TO TRACK STEADILY
WEST-NORTHWESTWARD THROUGH THE DURATION OF THE FORECAST PERIOD
ALONG THE SOUTHERN PERIPHERY OF THE LOW- TO MID-LEVEL STR CENTERED
TO THE NORTH. THE SYSTEM IS EXPECTED TO MAKE LANDFALL ALONG THE
CENTRAL VIETNAM COASTLINE AROUND TAU 24, TO THE SOUTH OF DA NANG.
CONDITIONS ARE GENERALLY FAVORABLE FOR INTENSIFICATION WITH THE
ONLY LIMITING FACTORS BEING THE MODEST, SINGLE-CHANNEL OUTFLOW AND
LIMITED TIME AVAILABLE PRIOR TO LANDFALL. TD 21W IS EXPECTED TO
INTENSIFY AT A CLIMATOLOGICAL RATE TO A PEAK OF 35 KNOTS AT OR JUST
PRIOR TO LANDFALL NEAR TAU 24. ONCE INLAND THE SYSTEM WILL RAPIDLY
DISSIPATE BY TAU 48 OVER EASTERN THAILAND DUE TO TERRAIN
INTERACTION. 

MODEL DISCUSSION: DUE TO THE RAPID DEVELOPMENT OF TD 21W, THERE IS
LITTLE AVAILABLE DETERMINISTIC TRACK GUIDANCE AVAILABLE, WITH THE
NAVGEM AND GFS THE ONLY AVAILABLE TRACKERS AT 1200Z. THESE TWO
MODELS ARE TIGHTLY GROUPED ON A NORTHWESTWARD TRACK, MAKING
LANDFALL BY TAU 24, HOWEVER THE NAVGEM INDICATES A LANDFALL ABOUT
75 NM FURTHER NORTH UP THE COAST THAN THE GFS. THE GFS ENSEMBLE AND
ECMWF ENSEMBLE MEMBERS ALSO AGREE ON A NORTHWESTWARD TRACK, BUT
SHOW A LANDFALL A BIT FURTHER NORTH, AT OR JUST SOUTH OF DA NANG.
IN LIGHT OF THE LACK OF DISCREET TRACK GUIDANCE, THERE IS ONLY
MEDIUM CONFIDENCE IN THE JTWC FORECAST TRACK, WHICH CLOSELY TRACKS
THE GFS FORECAST. SIMILARLY, THERE IS LITTLE TO NO DISCREET
INTENSITY GUIDANCE FOR THIS FORECAST, AND THE INTENSITY FORECAST IS
THUS BASED SOLELY ON ENVIRONMENTAL ANALYSIS, GLOBAL MODEL FIELDS
AND ENSEMBLE ESTIMATES, AND THUS THERE IS LOW CONFIDENCE IN THE
JTWC INTENSITY FORECAST.  

FORECAST CONFIDENCE:
   TRACK 0 - 72 HR: MEDIUM
   INTENSITY 0 - 72 HR: LOW//
NNNN